Piezoceramic stacks, also referred to as piezo stacks, which are commonly used in drives of fuel injectors, are commonly composed of a multiplicity of piezoelectric elements, for example ceramic layers, which are stacked one above the other. Commercially available piezo stacks commonly have more than 300 such ceramic layers. To generate a change in length of the piezo stack, the so-called stroke, the elements are subjected to an electric field. The piezoelectric effect has the effect here that the piezoceramic expands. The sum of the expansions of the individual piezo stack layers yields the stroke of the piezo stack as a whole. The electric field is generated by way of so-called inner electrodes which are arranged above and below each individual ceramic layer. Thus, a positive and a negative inner electrode are situated in alternating fashion between the individual ceramic layers, said electrodes being cyclically charged with electrical charge and discharged again. The contacting of the inner electrodes is realized generally by way of an external metallic coating on side faces of the piezo stack. The external metallic coating makes contact here in each case with only every second inner electrode, offset in each case by one electrode in relation to the opposite external metallic coating.
With the cyclic expansion of the piezo stack, expansion cracks arise in the ceramic and in the external metallic coating over the service life of the piezo component as a whole. For example, a common number of length change cycles over the service life of a piezo actuator amounts to more than 109 cycles. The cracks that form can lead to a disruption of the electrical contacting or of the inner electrodes suspended via the cracks, and thus impair or even prevent the change in length of the piezo stack.
Each active layer, that is to say each positive and negative inner electrode, which is intended to generate an electrical field for the ceramic layer situated in between, should therefore, where possible, be permanently contacted. This prerequisite should be satisfied even in the edge zones at the upper and lower end faces of the piezo stack. Since the piezo stack end faces generally exhibit mechanical contact with adjoining components composed of metal, the remaining spacing for electrical insulation with respect to the adjoining metallic components is often very small. A requirement with regard to the contacting is therefore very highly precise positioning during the assembly process, in order that it is thus preferably ensured that a subsequently applied insulation insulates both the contacting of the inner electrodes and the side faces of the piezo stack in a reliable manner in terms of the process and to an adequate extent.
Normally, the contact points between the ceramic inner electrodes or the external metallic coating and a contacting attached thereto are sensitive with regard to mechanical loads, for example tensile loads, shear forces or vibrations. In general, it is therefore demanded that such loads on the contact points at the ceramic surface be avoided. However, the avoidance of such loads is often not possible.
FIG. 17 and FIG. 18 shows a piezo component 10 according to the prior art.
The piezo component 10 has a piezo stack 12 which is electrically contacted at two mutually opposite side faces 14 by way of an external metallic coating 16. Here, the contacting 18 is realized by way of a first contacting element 20, which is applied directly to the external metallic coating 16, and a second contacting element 22 which, in the upper region of the piezo stack 12, is in direct contact with the first contacting element 20 and which extends beyond the piezo stack 12 in order that it can thus be contacted from the outside.
As can be seen in FIG. 18, the second contacting element 22 can be acted on by forces from the outside, specifically for example axial forces 24, radial forces 26, lateral forces 28 and torsional forces 30. Since the second contacting element 22 is directly connected to the first contacting element 20 in the circled region, such forces are transmitted directly to the first contacting element 20. This can result, in particular in the upper region of the piezo stack 12, in detachment of the contacting element 20. The forces may however also propagate further downward in the first contacting element 20 and lead to undesired detachments of the contacting 18 there too.